The invention relates in general to automatic gain control of a system utilizing measured environment parameters to define gain control to be applied, and more particularly to a laser scanning system which automatically controls the laser light intensity directed to an object to be detected in such a manner that the laser light reflected from that object received by the receiver is maintained within the dynamic (gray scale detection range of the receiver.
Specialized viewing systems can greatly enhance the observation of objects in turbid media; For example, such systems are useful in seawater for salvage and recovery operations or for object detection.
With the advent of the laser, relatively high powered directed illumination of a scene for imaging purposes becomes realizable.
Turbid media viewing systems utilizing laser technology are designed to avoid the deleterious effects of scattering, that is, to separate information containing light from non-information containing light. Conventionally, a method of accomplishing this is to separate the light source from the receiver. The greater the separation, the less scattered light is detected by the receiver. Usually, the source and receiver are as far apart as is permitted by the size of the vehicle upon which they are mounted.
An underwater viewing system can be found in U.S. Pat. No. 4,707,128 by inventor Bryan W. Coles. In this system an emitter, such as a pulsed laser, emits a short pulse of light which travels to a beam splitter where part of the light is transmitted and part is deflected. The transmitted portion passes through optics and is deflected by an X-Y scanning element in such a manner that a portion of an object is illuminated in a point-by-point manner. Light reflected from the object travels back along a path that is coaxial with the path of the transmitted light pulse striking the scanning elements and the beam splitter. The return light deflected by the beam splitter is received by a light sensor, such as a photomultiplier tube.
Other prior art systems consists of similar devices in which an image is manually adjusted, either real-time or based on empirical charts, to achieve an acceptable image at the receiving device.
Alternatively, the prior art also consists of using Automatic Gain Control (AGC), which consists of feeding back an error signal from the received image to adjust the receiver gain to within its dynamic (gray scale) range. This type of derived AGC relies on a historical average for a baseline, requiring a receiving device with a larger detector dynamic range than required of the system of the instant invention to accommodate potential signal drop-outs or spikes.
In some of these state of the art viewing systems, manual adjustments are made consisting of adjusting the gain on a monitor displaying the image in real-time to obtain a picture with discernable intensity levels. This method of maintaining a suitable received image requires the presence of a human operator constantly making adjustments for maximum image quality.
Another method of maintaining a received light image in the dynamic range of the receiving device is through the use of error signal feedback. In this method, the average value of the scene being imaged is fed back to the receiver gain controller. The receiver gain is controlled such that the average value is maintained within the receiver dynamic range. The time constant on such a prior art device can be preset to roughly accommodate the scan angle of the device.
Neither of these two methods incorporates any time dependency in the gain function.
There has not been an automatic means for real time control of the light output intensity of a laser light scanning system in which the intensity of reflected light from an illuminated object is maintained within the dynamic light intensity range of the image receiver until the emergence of the instant invention.